Power devices are widely used to carry large currents and/or support high voltages. Conventional power devices are generally fabricated from monocrystalline silicon semiconductor material. One type of power device is the bipolar junction transistor (BJT). The BJT is a well-known and frequently used power device. A BJT typically includes a semiconductor material having two opposing p-n junctions in close proximity to one another. Thus, BJTs may be referred to as “n-p-n” or “p-n-p” transistors.
In operation, current carriers enter a region of the semiconductor material of a first conductivity type adjacent one of the p-n junctions, which is called the emitter. Most of the charge carriers exit the device from a region of the semiconductor material of the first conductivity type adjacent the other p-n junction, which is called the collector. A small portion of semiconductor material known as the base, having a second conductivity type, opposite the first conductivity type of the collector and the emitter, is positioned between the collector and the emitter. The two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter.
When current is injected into or extracted from the base, depending upon whether the BJT is n-p-n or p-n-p, the flow of charge carriers, i.e., electrons or holes, which can move from the emitter to the collector, may be affected. Typically, small currents applied to the base can control proportionally larger currents passing through the BJT, giving it usefulness as a component of electronic circuits. Structural and operational details of BJT's are discussed in Solid State Electronic Devices by B. Streetman (2nd edition (1980), chapter 7), the content of which is incorporated herein by reference as if set forth in its entirety.
The material that makes up a device can contribute to the operability and usefulness of the device. For example, conventional BJTs are typically formed of silicon (Si), but can also include gallium arsenide (GaAs) and indium phosphide (InP). Silicon carbide (SiC) has also been used as a material for BJTs. SiC has potentially advantageous semiconductor characteristics, for example, a wide bandgap, high electric field breakdown strength, high thermal conductivity, high melting point and high-saturated electron drift velocity. Thus, relative to devices formed in other semiconductor materials, for example, Si, electronic devices formed in SiC may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under high radiation densities.
Due to their ability to function at high frequencies, high temperatures, and/or high power levels, SiC transistors can be highly desirable for use in applications such as high frequency transmitters for radar and communications, for high power switching applications, and for high temperature operations such as jet engine control. Accordingly, methods of producing device quality SiC and devices formed from SiC have been of interest to scientists and engineers for several decades.
SiC crystallizes in over 150 different polytypes, or crystal structures, of which the most common are designated 3C, 4H, and 6H where “C” stands for “cubic” and “H” for “hexagonal.” SiC generally is a difficult material to use in semiconductor devices. In particular, the high melting point of SiC may render techniques, such as alloying and diffusion of dopants, more difficult, because a number of the other materials necessary to perform such operations may tend to break down at the high temperatures used to affect SiC. SiC is also an extremely hard material, and indeed a common use for SiC is as an abrasive. Notwithstanding these potential difficulties, junctions, diodes, transistors, and other devices have been fabricated from SiC.
SiC BJTs are discussed in, for example, U.S. Pat. No. 4,945,394 to Palmour et al., the disclosure of which is incorporated herein by reference as if set forth in its entirety. As discussed therein, SiC BJTs have a base and an emitter formed as wells using high temperature ion implantation. Furthermore, SiC BJTs are also discussed in U.S. Pat. No. 6,218,254 to Singh et al., the disclosure of which is incorporated herein by reference as if set forth in its entirety. As discussed therein, SiC BJTs having a base and an emitter are formed using epitaxial growth and high temperature ion implantation. Singh further discusses the use of removable dielectric spacers to improve the alignment of base and emitter regions.
SiC BJTs may also include at least one highly doped implant region for the base and/or the emitter. The formation of the implant region may cause damage to the SiC crystal. For example, the current gain of the device may be reduced due to surface recombination current due to implant-induced defects. The current gain of the device may be reduced as a function of time due to stacking faults in the base region, which may result from the defects created by the implantation process. Furthermore, the highly doped emitter regions present in these devices may have a poor lifetime, which may further reduce the current gain of the device.
Referring now to FIG. 1, a cross section of a conventional SiC BJT will be discussed. As illustrated in FIG. 1, an N− SiC epitaxial (epi) collector layer 116 is provided on a highly doped N+ SiC substrate 114. The N+ SiC substrate 114 and the N− SiC epi collector layer 116 can define a collector 110 for the device. A p-type (P) SiC epi base layer 120 is provided on the N− SiC epi collector layer 116. As further illustrated in FIG. 1, an n-type epi emitter mesa 130 (N+) is provided on the p-type SiC base layer 120. The emitter mesa 130 may be formed by selective epitaxial growth and/or epitaxial growth and subsequent etching. Moreover, highly doped (P+) regions 124 may be formed in the base layer 120 by implantation techniques. In particular, the implantation of the P+ base regions 124 may be carried out at from about 650° C. to about 1100° C. using aluminum ions (Al+) having energy ranging from about 20 keV to about 360 keV. As discussed above, this implant may cause damage to the SiC crystal, which may not be entirely removed by a high temperature (1600-1700° C.) activation anneal.
In particular, the defects introduced by the implant may result in surface recombination current and stacking fault formation in the base layer of the device. This relationship is represented by the equation:β=iC/iBwhere β represents current gain, iC represents collector current and iB represents base current. Base current iB may be further represented byiB=iBr+iBi+iBsr;where iBr is recombination current, iBi represents majority carriers injected across the emitter junction, and iBsr represents surface recombination current. Current gain may be represented by the equation:β=iC/(iBr+iBi+iBsr).Accordingly, any increase in surface recombination current iBsr in the base region may yield a lower current gain for the device.
Referring again to FIG. 1, a dielectric passivation layer 140 is provided on the surface of the device. The dielectric passivation layer 140 may comprise silicon dioxide and/or silicon nitride. The BJT further includes base (B) contacts 122, an emitter (E) contact 132 and a collector (C) contact 112. The highly doped regions designated as N+ 114, 130 and P+ 124 may be provided to improve the quality of the ohmic contacts. The base emitter and collector contacts 122, 132 and 112, respectively, may include multilayer structures including ohmic metal and/or overlaying layer metals. It will also be understood that other embodiments of bipolar transistors may eliminate one or more of the layers and/or regions shown in FIG. 1, or may add additional regions and/or layers.
Unfortunately, it has been found that the current gain of SiC BJTs, such as illustrated in FIG. 1, may degrade over time under typical operating conditions. In particular, FIGS. 2A-2D illustrate output characteristics of an SiC BJT with increasing cumulative stress at 10 A for 0 minutes, 15 minutes, 30 minutes and 16 hours, respectively. Current gain degradation is shown within 15 minutes of operation and continued degradation is shown by the succeeding output characteristics of FIGS. 2A-2D.